We could make attachment support a widget that users could choose to insert into a page. When should a feature be done as an always-there part of the core UX vs. an optionally-there widget?
Take tagging for example. Every piece of content has a tag box just under the title, showing the current tags and allowing a new tag. That could be replaced with a widget. If the author of a piece of content wanted tags on their page, they would include the Tag Widget somewhere in their document.
That is likely to be viewed unfavorably. The basics of tagging is a central part of the system. That support needs to always be there, in the same spot, with the same behavior.
Attachments are less “optional” than tagging. Making pages an attractive alternative to file folders is the central part of the WNG experience. We need to ensure that it is always visible, as many of the users won’t know or won’t use widgets. Stated differently, most won’t even know attachment support exists.
We should, however, limit the core UX for attachments and allow widgets that add extra power to the attached files.